According to “World Health Report” issued by WHO in 1996, malaria is, like tuberculosis and diphtheria, an infectious disease which once had been considered to have already been overcome, but which is now designated as one of reemerging infections in view of such present situation that there again appear signs of a prevalence of malaria. In 1995, the number of deaths from malaria amounted to 2.1 millions in the world, and presently the number of people as infected with malarial parasites are said to have amounted to 270 millions in total. Areas in which the infection with malaria occurs are not only in the tropics, but also are now spreading, due to the phenomenon of global warming, over the temperate regions, where the malarial infections were rarely found in the past. Even in the Main Island of Japan, there occurs some fear of prevailing malarial diseases in a near future.
The malarial parasites, with which human beings can be infected, include Plasmodium falciparum; Plasmodium vivax; Plasmodium malariae; and Plasmodium ovate which belong to Sporozoea. Further, there are included those malarial parasites which can infect apes and murines.
Some strains of the malarial parasites have gained a tolerance to a variety of the known antimalarials and thus are difficult to be eliminated. In these circumstances, there have been made extensive investigations for extermination of the malarial diseases, including the development of vaccine, elucidation of ecology and physiology of mosquitoes carrying the malarial diseases, elucidation of physiological mechanism of the malarial parasites, analysis of immune responses of human beings against the malarial parasites, analysis of pathophysiology of malaria, as well as development of new medicines for treating malaria, and so on. In particular, there now occurs a keen demand for developing new antimalarials effective to completely exterminate malaria.
As the medicines currently utilized for treating malaria, there are already known quinine, chloroquine, pyrimethamine, mefloquine, artemisinin, and so on. However, all these known antimalarials are not necessarily deemed to be satisfactory antimalarials since they have poor reliability of their medicinal effects as well as their significant toxicity and side effects against the human beings.
There is further known some polyether-type antibiotics which are produced by microorganisms and which have an antimalarial activity. These antibiotics include monensin and nigericin [refer to Life Sci., Vol.59(20), pp. 309-315 (1996)] as well as salinomycin [refer to Zentralbl. Bakteriol., Mikrobiol. Hyg., Ser. A 256(3), pp. 305-313 (1984)] However, all of monensin, nigericin and salinomycin are of high acute toxicity upon oral administrations of them and also are low and instable in respect of their antimalarial effects, so that they have not yet been utilized in practical applications by now.
There is a further known polyether-type antibiotic, namely SF2487 substance which is disclosed in Japanese Patent No. 1725905 (Japanese Patent Publication Hei-4-13353) to have an anti-microbial activity and an anti-influenza virus activity. This antibiotic SF2487 substance and sodium salt, potassium salt and Ag salt thereof are disclosed also in “The Journal of Antibiotics”, Vol. XLIII, No. 3, pp.259-266 (March, 1990), along with its chemical structural formula and the process for the preparation of the SF2487 substance.